The food industry uses numerous bacteria, in particular lactic acid bacteria, in order to improve the taste and the texture of foods but also in order to extend the shelf life of these foods. In the case of the dairy industry, lactic acid bacteria are used intensively in order to bring about the acidification of milk (by fermentation) but also in order to texturize the product into which they are incorporated.
Among the lactic acid bacteria used in the food industry, there can be mentioned the genera Streptococcus, Lactococcus, Lactobacillus, Leuconostoc, Pediococcus and Bifidobacterium. The lactic acid bacteria of the species Streptococcus thermophilus are used extensively alone or in combination with other bacteria such as Lactobacillus delbrueckii subsp. bulgaricus for the production of food products, in particular fermented products. They are used in particular in the formulation of the ferments used for the production of fermented milks, for example yoghurts. Certain of them play a dominant role in the development of the texture of the fermented product. This characteristic is closely linked to the production of polysaccharides.
The current trend in yoghurts is for mild flavor and high texture. Today this is achieved by the use of cultures which produce a mild flavor and the addition of thickeners or protein to give the desired thickness. Yoghurt producers would like to be able to make yoghurt with these properties without the addition of thickening agents. This will help them reduce cost and give a cleaner label. One very attractive way to achieve this would be to have a starter culture which produces a high level of texture.
In order to meet the requirements of the industry, it has become necessary to provide novel texturizing strains of lactic acid bacteria, in particular of Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus, for texturizing food products. Especially there is a need for novel texturizing strains of Lactobacillus delbrueckii subsp bulgaricus which can be used together with novel texturizing strains of Streptococcus thermophilus. 
The inventors of the present invention has previously developed a novel selection method for the identification of improved lactic acid bacteria, such as Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus bacteria, which result in high texture when used for fermenting milk substrate for fermented milk products.
The method, which is described in International (PCT) patent application No. WO 2012/052557, relates to that the present inventors have identified a surprisingly relevant link between resistance to D-cycloserine and functionally equivalent antibiotics and improved texturizing properties of the lactic acid bacteria.
D-cycloserine (D-4-amino-isoxasolidone) is an antibiotic which inhibits alanine racemase, D-alanyl-D-alanine ligase, D-alanyl-alanine synthase and D-alanine permease causing cell lysis. D-alanine racemase is essential for the production of D-alanine, an integral part of the peptidoglycan layer of the cell wall.
Ampicillin is an antibiotic of the beta-lactam class of antibiotics and is effective against many gram-positive bacteria including most lactic acid bacteria. Ampicillin is a competitive inhibitor of enzymes of the DD-transpeptidase type (EC 3.4.16.4). Inhibition of DD-transpeptidase by ampicillin prevents the formation of a peptide bond required for the formation of the bacterial cell wall and ultimately leads to lysis of the cell.
Mutants resistant to ampicillin have been described for a number of different bacteria but—to the knowledge of the present inventors—there is in the prior art not described or suggested any herein relevant link between resistance to ampicillin and improved texturizing properties.